1. Field of the Invention
The present invention relates to an agricultural harvesters such as combines, and, more particularly, to an agricultural harvester having an additional weight transfer axle that is steerable in coordination with the rear steering axle.
2. Description of the Related Art
An agricultural harvester known as a “combine” is historically termed such because it combines multiple harvesting functions with a single harvesting unit, such as picking, threshing, separating and cleaning A combine includes a header which removes the crop from a field, and a feeder housing which transports the crop matter into a threshing rotor. The threshing rotor rotates within a perforated housing, which may be in the form of adjustable concaves, and performs a threshing operation on the crop to remove the grain. Once the grain is threshed it falls through perforations in the concaves and is transported to a grain pan. From the grain pan the grain is cleaned using a cleaning system, and is then transported to a grain tank onboard the combine. The cleaning system includes a cleaning fan which blows air through oscillating sieves to discharge chaff and other debris toward the rear of the combine. Non-grain crop material such as straw from the threshing section proceeds through a straw chopper and out the rear of the combine. When the grain tank becomes full, the combine is positioned adjacent a vehicle into which the grain is to be unloaded, such as a semi-trailer, gravity box, straight truck, or the like; and an unloading system on the combine is actuated to transfer the grain into the vehicle.
In order to support the weight of the header, which is cantilevered forward of the combine, and in order to support the weight of the machinery and grain load of the combine itself, combines use a heavy solid non-steering front axle. In order to provide steering, the rear axle of the combine is a steerable axle, which is commonly actuated by one or more hydraulic actuators. This rear steering arrangement further provides excellent maneuverability of the combine header, allowing an operator to easily align the header with the crop being harvested.
However, when the header is removed from the combine, such as to configure the combine for on-road travel, a significant amount of weight is transferred to the rear steering axle. While the capacity of this rear steering axle is adequate for field operations, even with the header removed from the combine, the increasing size and weight of combines has resulted in rear steering axle loads with the header of the combine removed that are at or above the maximum axle weight allowable on road according to regulations. Furthermore, the overall combine may be at or above the maximum weight allowable on road for a vehicle with two axles.
In order to compensate for this, it is known to provide an additional rear axle, such as a tag axle, behind the rear steering axle of a combine. This tag axle, or weight transfer axle, may be lowered to engage the ground and provide additional support and weight distribution, particularly when the header is removed from the combine. The weight transfer axle may further be raised when the header is attached to the combine and the additional support and weight distribution is not needed. The weight transfer axle may be passively steerable, such that it is built with caster so that when the combine is steered using the rear steering axle, the weight transfer axle wheels passively swivel to follow the rear steering axle. However, this arrangement is susceptible to wear and tear, due at least in part to the weight transfer axle passively following the rear steering axle imperfectly, resulting in tire scrub and high forces on the weight transfer axle and its support structure. Furthermore, the weight transfer axle must be lifted when the combine is moved in a rearward direction, as the same caster geometry that allows the weight transfer axle wheels to passively swivel and follow the rear steering axle when moving forward, causes the weight transfer axle wheels to swivel inappropriately when the combine is moved in a rearward direction.
What is needed is a method of controllably steering a weight transfer axle. The method needs to be robust and easily adaptable to existing vehicle structures. The method needs to further accommodate articulation of the rear steering axle and weight transfer axle, while allowing the weight transfer axle to be raised and lowered as required.